Multivariate optical elements for optical analysis system

ABSTRACT

A method of developing a multivariate optical element for an optical analysis system includes forming an optically absorptive spectral element having an optically absorptive material, the optically absorptive material being absorbing in a predetermined spectral region; and utilizing the optically absorptive spectral element in the optical analysis system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 60/740,170, filed Nov. 28, 2005;incorporated herein by reference in its entirety.

BACKGROUND OF THE DISCLOSURE

Light conveys information through data. When light interacts withmatter, for example, it carries away information about the physical andchemical properties of the matter. A property of the light, for example,its intensity, may be measured and interpreted to provide informationabout the matter with which it interacted. That is, the data carried bythe light through its intensity may be measured to derive informationabout the matter. Similarly, in optical communications systems, lightdata is manipulated to convey information over an optical transmissionmedium, for example fiber optic cable. The data is measured when thelight signal is received to derive information.

In general, a simple measurement of light intensity is difficult toconvert to information because it likely contains interfering data. Thatis, several factors may contribute to the intensity of light, even in arelatively restricted wavelength range. It is often impossible toadequately measure the data relating to one of these factors since thecontribution of the other factors is unknown.

It is possible, however, to derive information from light. An estimatemay be obtained, for example, by separating light from several samplesinto wavelength bands and performing a multiple linear regression of theintensity of these bands against the results of conventionalmeasurements of the desired information for each sample. For example, apolymer sample may be illuminated so that light from the polymer carriesinformation such as the sample's ethylene content. Light from each ofseveral samples may be directed to a series of bandpass filters whichseparate predetermined wavelength bands from the light. Light detectorsfollowing the bandpass filters measure the intensity of each light band.If the ethylene content of each polymer sample is measured usingconventional means, a multiple linear regression of ten measuredbandpass intensities against the measured ethylene content for eachsample may produce an equation such as:y=a ₀ +a ₁ w ₁ +a ₂ w ₂ + . . . +a ₁₀ w ₁₀  (“Equation 1”)where y is ethylene content, a_(n) are constants determined by theregression analysis, and w_(n) is light intensity for each wavelengthband.

Equation 1 may be used to estimate ethylene content of subsequentsamples of the same polymer type. Depending on the circumstances,however, the estimate may be unacceptably inaccurate since factors otherthan ethylene may affect the intensity of the wavelength bands. Theseother factors may not change from one sample to the next in a mannerconsistent with ethylene.

A more accurate estimate may be obtained by compressing the data carriedby the light into principal components. To obtain the principalcomponents, spectroscopic data is collected for a variety of samples ofthe same type of light, for example from illuminated samples of the sametype of polymer. For example, the light samples may be spread into theirwavelength spectra by a spectrograph so that the magnitude of each lightsample at each wavelength may be measured. This data is then pooled andsubjected to a linear-algebraic process known as singular valuedecomposition (SVD). SVD is at the heart of principal componentanalysis, which should be well understood in this art. Briefly,principal component analysis is a dimension reduction technique, whichtakes m spectra with n independent variables and constructs a new set ofeigenvectors that are linear combinations of the original variables. Theeigenvectors may be considered a new set of plotting axes. The primaryaxis, termed the first principal component, is the vector, whichdescribes most of the data variability. Subsequent principal componentsdescribe successively less sample variability, until only noise isdescribed by the higher order principal components.

Typically, the principal components are determined as normalizedvectors. Thus, each component of a light sample may be expressed asx_(n) z_(n), where x_(n) is a scalar multiplier and z_(n) is thenormalized component vector for the n_(th) component. That is, z_(n) isa vector in a multi-dimensional space where each wavelength is adimension. As should be well understood, normalization determines valuesfor a component at each wavelength so that the component maintains itshape and so that the length of the principal component vector is equalto one. Thus, each normalized component vector has a shape and amagnitude so that the components may be used as the basic buildingblocks of all light samples having those principal components.Accordingly, each light sample may be described in the following formatby the combination of the normalized principal components multiplied bythe appropriate scalar multipliers:x₁z₁+x₂z₂+ . . . +x_(n)z_(n).

The scalar multipliers x_(n) may be considered the “magnitudes” of theprincipal components in a given light sample when the principalcomponents are understood to have a standardized magnitude as providedby normalization.

Because the principal components are orthogonal, they may be used in arelatively straightforward mathematical procedure to decompose a lightsample into the component magnitudes, which accurately describe the datain the original sample. Since the original light sample may also beconsidered a vector in the multi-dimensional wavelength space, the dotproduct of the original signal vector with a principal component vectoris the magnitude of the original signal in the direction of thenormalized component vector. That is, it is the magnitude of thenormalized principal component present in the original signal. This isanalogous to breaking a vector in a three dimensional Cartesian spaceinto its X, Y and Z components. The dot product of the three-dimensionalvector with each axis vector, assuming each axis vector has a magnitudeof 1, gives the magnitude of the three dimensional vector in each of thethree directions. The dot product of the original signal and some othervector that is not perpendicular to the other three dimensions providesredundant data, since this magnitude is already contributed by two ormore of the orthogonal axes.

Because the principal components are orthogonal, or perpendicular, toeach other, the dot, or direct, product of any principal component withany other principal component is zero. Physically, this means that thecomponents do not interfere with each other. If data is altered tochange the magnitude of one component in the original light signal, theother components remain unchanged. In the analogous Cartesian example,reduction of the X component of the three dimensional vector does notaffect the magnitudes of the Y and Z components.

Principal component analysis provides the fewest orthogonal componentsthat can accurately describe the data carried by the light samples.Thus, in a mathematical sense, the principal components are componentsof the original light that do not interfere with each other and thatrepresent the most compact description of the entire data carried by thelight. Physically, each principal component is a light signal that formsa part of the original light signal. Each has a shape over somewavelength range within the original wavelength range. Summing theprincipal components produces the original signal, provided eachcomponent has the proper magnitude.

The principal components comprise a compression of the data carried bythe total light signal. In a physical sense, the shape and wavelengthrange of the principal components describe what data is in the totallight signal while the magnitude of each component describes how much ofthat data is there. If several light samples contain the same types ofdata, but in differing amounts, then a single set of principalcomponents may be used to exactly describe (except for noise) each lightsample by applying appropriate magnitudes to the components.

The principal components may be used to accurately estimate informationcarried by the light. For example, suppose samples of a certain brand ofgasoline, when illuminated, produce light having the same principalcomponents. Spreading each light sample with a spectrograph may producewavelength spectra having shapes that vary from one gasoline sample toanother. The differences may be due to any of several factors, forexample differences in octane rating or lead content.

The differences in the sample spectra may be described as differences inthe magnitudes of the principal components. For example, the gasolinesamples might have four principal components. The magnitudes x_(n) ofthese components in one sample might be J, K, L, and M, whereas in thenext sample the magnitudes may be 0.94 J, 1.07K, 1.13 L and 0.86M. Asnoted above, once the principal components are determined, thesemagnitudes exactly describe their respective light samples.

Refineries desiring to periodically measure octane rating in theirproduct may derive the octane information from the component magnitudes.Octane rating may be dependent upon data in more than one of thecomponents. Octane rating may also be determined through conventionalchemical analysis. Thus, if the component magnitudes and octane ratingfor each of several gasoline samples are measured, a multiple linearregression analysis may be performed for the component magnitudesagainst octane rating to provide an equation such as:y=a ₀ +a ₁ x ₁ +a ₂ x ₂ +a ₃ x ₃ +a ₄ x ₄  (“Equation 2”)where y is octane rating, a_(n) are constants determined by theregression analysis, and x₁, x₂, x₃ and x₄ are the first, second, thirdand fourth principal component magnitudes, respectively.

Using Equation 2, which may be referred to as a regression vector,refineries may accurately estimate octane rating of subsequent gasolinesamples. Conventional systems perform regression vector calculations bycomputer, based on spectrograph measurements of the light sample bywavelength. The spectrograph system spreads the light sample into itsspectrum and measures the intensity of the light at each wavelength overthe spectrum wavelength range. If the regression vector in the Equation2 form is used, the computer reads the intensity data and decomposes thelight sample into the principal component magnitudes x_(n) bydetermining the dot product of the total signal with each component. Thecomponent magnitudes are then applied to the regression equation todetermine octane rating.

To simplify the procedure, however, the regression vector is typicallyconverted to a form that is a function of wavelength so that only onedot product is performed. Each normalized principal component vectorz_(n) has a value over all or part of the total wavelength range. Ifeach wavelength value of each component vector is multiplied by theregression constant a_(n) corresponding to the component vector, and ifthe resulting weighted principal components are summed by wavelength,the regression vector takes the following form:y=a ₀ +b ₁ u ₁ +b ₂ u ₂ + . . . +b _(n) u _(n)   (“Equation 3”)where y is octane rating, a₀ is the first regression constant fromEquation 2, b_(n) is the sum of the multiple of each regression constanta_(n) from Equation 2 and the value of its respective normalizedregression vector at wavelength n, and u_(n) is the intensity of thelight sample at wavelength n. Thus, the new constants define a vector inwavelength space that directly describes octane rating. The regressionvector in a form as in Equation 3 represents the dot product of a lightsample with this vector.

Normalization of the principal components provides the components withan arbitrary value for use during the regression analysis. Accordingly,it is very unlikely that the dot product result produced by theregression vector will be equal to the actual octane rating. The numberwill, however, be proportional to the octane rating. The proportionalityfactor may be determined by measuring octane rating of one or moresamples by conventional means and comparing the result to the numberproduced by the regression vector. Thereafter, the computer can simplyscale the dot product of the regression vector and spectrum to produce anumber approximately equal to the octane rating.

In a conventional spectroscopy analysis system, a laser directs light toa sample by a bandpass filter, a beam splitter, a lens and a fiber opticcable. Light is reflected back through the cable and the beam splitterto another lens to a spectrograph. The spectrograph separates light fromthe illuminated sample by wavelength so that a detection device such asa charge couple detector can measure the intensity of the light at eachwavelength. The charge couple detector is controlled by controller andcooled by a cooler. The detection device measures the light intensity oflight from the spectrograph at each wavelength and outputs this datadigitally to a computer, which stores the light intensity over thewavelength range. The computer also stores a previously derivedregression vector for the desired sample property, for example octane,and sums the multiple of the light intensity and the regression vectorintensity at each wavelength over the sampled wavelength range, therebyobtaining the dot product of the light from the substance and theregression vector. Since this number is proportional to octane rating,the octane rating of the sample is identified.

Since the spectrograph separates the sample light into its wavelengths,a detector is needed that can detect and distinguish the relativelysmall amounts of light at each wavelength. Charge couple devices providehigh sensitivity throughout the visible spectral region and into thenear infrared with extremely low noise. These devices also provide highquantum efficiency, long lifetime, imaging capability and solid-statecharacteristics. Unfortunately, however, charge couple devices and theirrequired operational instrumentation are very expensive. Furthermore,the devices are sensitive to environmental conditions. In a refinery,for example, they must be protected from explosion, vibration andtemperature fluctuations and are often placed in protective housingsapproximately the size of a refrigerator. The power requirements,cooling requirements, cost, complexity and maintenance requirements ofthese systems have made them impractical in many applications.

Multivariate optical computing (MOC) is a powerful predictivespectroscopic technique that incorporates a multi-wavelength spectralweighting directly into analytical instrumentation. This is in contrastto traditional data collection routines where digitized spectral data ispost processed with a computer to correlate spectral signal with analyteconcentration. Previous work has focused on performing such spectralweightings by employing interference filters called Multivariate OpticalElements (MOE(s)). Other researchers have realized comparable results bycontrolling the staring or integration time for each wavelength duringthe data collection process. All-optical computing methods have beenshown to produce similar multivariate calibration models, but themeasurement precision via an optical computation is superior to atraditional digital regression.

MOC has been demonstrated to simplify the instrumentation and dataanalysis requirements of a traditional multivariate calibration.Specifically, the MOE utilizes a thin film interference filter to sensethe magnitude of a spectral pattern. A no-moving parts spectrometerhighly selective to a particular analyte may be constructed by designingsimple calculations based on the filter transmission and reflectionspectra. Other research groups have also performed optical computationsthrough the use of weighted integration intervals and acousto-opticaltunable filters, digital mirror arrays and holographic gratings.

The measurement precision of digital regression has been compared tovarious optical computing techniques including MOEs, positive/negativeinterference filters and weighted-integration scanning opticalcomputing. In a high signal condition where the noise of the instrumentis limited by photon counting, optical computing offers a highermeasurement precision when compared to its digital regressioncounterpart. The enhancement in measurement precision for scanninginstruments is related to the fraction of the total experiment timespent on the most important wavelengths. While the detector integratesor coadds measurements at these important wavelengths, the signalincreases linearly while the noise increases as a square root of thesignal. Another contribution to this measurement precision enhancementis a combination of the Felgott's and Jacquinot's advantage, which ispossessed by MOE optical computing.

BRIEF SUMMARY OF THE DISCLOSURE

The present disclosure is directed generally to an optical system formultivariate optical computing (MOC), which is generally described inU.S. Pat. No. 6,198,531 B1 to Myrick et al. and in U.S. Pat. No.6,529,276 B1 to Myrick as a predictive spectroscopy technique thatincorporates a multi-wavelength spectral weighting directly intoanalytical instrumentation. Both of these patents are incorporatedherein for all purposes by reference thereto.

The present disclosure more particularly provides systems and methodsfor deriving information from light. For example, processes forselecting particular spectral elements and components for the opticalanalysis system are described herein.

According to one aspect of the disclosure a method of developing amultivariate optical element for an optical analysis system for use in apredetermined spectral region may include forming an opticallyabsorptive spectral element having an optically absorptive material, theoptically absorptive material defining a wavelength-dependent property;and disposing the optically absorptive spectral element in the opticalanalysis system.

According to this aspect of the disclosure, the optically absorptivematerial may be a layer in the optically absorptive spectral element.Further, the optically absorptive material may be a film; may betransmissive; may be reflective; and/or may be both transmissive andreflective. Still further, the optically absorptive material may be asilicon, a doped silicon, a germanium, a doped germanium, a sapphire,and/or a doped sapphire.

Also according to this aspect of the disclosure, the method may includeselecting the optically absorptive material based on the predeterminedspectral region.

According to another aspect of the disclosure, a method of developing amultivariate optical element for an optical analysis system for use in apredetermined spectral region may include forming a plurality ofoptically absorptive spectral elements each including an opticallyabsorptive material defining a wavelength-dependent property; anddisposing the optically absorptive spectral elements in the opticalanalysis system.

According to the exemplary method, the optically absorptive spectralelements may be transmissive or reflective. The optically absorptivematerial may also be a silicon or a doped silicon; and/or a germanium ora doped germanium; and/or a sapphire, or a doped sapphire.

According to yet another aspect of the disclosure, a multivariateoptical analysis system for use in a predetermined spectral region mayinclude a spectral element having an optically absorptive spectralmaterial, the optically absorptive material defining awavelength-dependent property. The spectral element may be amultivariate optical element.

According to this aspect of the disclosure, the optically absorptivematerial may be a layer in the spectral element. Still further, theoptically absorptive material may be a film. Additionally, the opticallyabsorptive material may be a silicon, a doped silicon, a germanium, adoped germanium, a sapphire, and/or a doped sapphire. Moreover, theoptically absorptive material may be transmissive, reflective or bothtransmissive and reflective.

The exemplary multivariate optical analysis system in this aspect mayfurther include a light source being configured to radiate a first lightalong a first ray path; a modulator disposed in the first ray path, themodulator being configured to modulate the first light to a desiredfrequency; an additional spectral element disposed proximate themodulator, the additional spectral element being configured to filterthe first light for a spectral range of interest of a sample; a cavitybeing configured to direct the first light in a direction of the sample,the cavity being configured to receive and direct a second lightgenerated by a reflection of the first light from the sample, the cavitybeing further configured to separate the first and second lights; abeamsplitter being configured to split the second light into a firstbeam and a second beam; wherein the optically absorptive spectralelement is disposed to receive the first beam, the optically absorptivespectral element being configured to optically filter data carried bythe first beam into at least one orthogonal component of the first beam;a first detector mechanism in communication with the opticallyabsorptive spectral element to measure a property of the orthogonalcomponent to measure the data; and a second detector mechanism beingconfigured to receive the second beam for comparison of the property ofthe orthogonal component to the second beam.

Additional objects and advantages of the present subject matter are setforth in, or will be apparent to those of ordinary skill in the art,from the detailed description herein. Also, it should be furtherappreciated that modifications and variations to the specificallyillustrated, referred and discussed features and elements hereof may bepracticed in various embodiments and uses of the disclosure withoutdeparting from the spirit and scope of the subject matter. Variationsmay include, but are not limited to, substitution of equivalent means,features, or steps for those illustrated, referenced, or discussed, andthe functional, operational, or positional reversal of various parts,features, steps, or the like.

Still further, it is to be understood that different embodiments, aswell as different presently preferred embodiments, of the presentsubject matter may include various combinations or configurations ofpresently disclosed features, steps, or elements, or their equivalents(including combinations of features, parts, or steps or configurationsthereof not expressly shown in the figures or stated in the detaileddescription of such figures). Additional embodiments of the presentsubject matter, not necessarily expressed in the summarized section, mayinclude and incorporate various combinations of aspects of features,components, or steps referenced in the summarized objects above, and/orother features, components, or steps as otherwise discussed in thisapplication. Those of ordinary skill in the art will better appreciatethe features and aspects of such embodiments, and others, upon review ofthe remainder of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present subject matter, includingthe best mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures, in which:

FIG. 1 is a schematic plan view of an optical analysis system accordingto an aspect of the disclosure;

FIG. 2 is a perspective view of an optical analysis system according toanother aspect of the disclosure;

FIG. 3 is a partial cut-away plan view of the optical analysis system asin FIG. 2;

FIG. 4 is partial cut-away plan view of a lamp side of the opticalanalysis system as in FIG. 2;

FIG. 5 is partial cut-away plan view of a detector side of the opticalanalysis system as in FIG. 2;

FIG. 6 is perspective view of an optical analysis system according toyet another aspect of the disclosure;

FIG. 7 is a table of exemplary detectors that may be used in the opticalanalysis system as in FIG. 1; and

FIG. 8 is a table of exemplary spectral elements that may be usedaccording to various aspects of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

Detailed reference will now be made to the drawings in which examplesembodying the present subject matter are shown. The detailed descriptionuses numerical and letter designations to refer to features of thedrawings. Like or similar designations of the drawings and descriptionhave been used to refer to like or similar parts of the disclosure.

The drawings and detailed description provide a full and writtendescription of the examples in the disclosure, and of the manner andprocess of making and using those examples, so as to enable one skilledin the pertinent art to make and use them, as well as the best mode ofcarrying out the examples. However, the examples set forth in thedrawings and detailed description are provided by way of explanationonly and are not meant as limitations of the disclosure. The presentdisclosure thus includes any modifications and variations of thefollowing examples as come within the scope of the appended claims andtheir equivalents.

With reference now to FIG. 1, an exemplary embodiment of the presentsubject matter is designated generally by reference number 10. As shown,the optical analysis system 10 broadly includes a housing 12, anillumination or light source 14, a chopper wheel 18, one or morespectral elements 20, a focusing lens 26, a beam splitter 28, a firstdetector assembly 30 including a multivariate optical element 48, and asecond detector assembly 32 including a detector 56. The opticalanalysis system 10 further includes an electrical connection 60, apressurization sensor 62 and a purge gas assembly 64, which thoseskilled in the art will readily understand; therefore, furtherdescription is not necessary to understand and practice these aspects ofthe disclosure.

Also shown in FIG. 1, the illumination source 14 provides a light 34,which passes through a collecting Fresnel lens 16A and into and throughthe spectral element(s) 20. In this example, the illumination source 14is rated for at least about 10,000 hours of operation, which alleviatesa need for redundant illumination sources though they may be provided ifdesired. Also in this example, the collecting Fresnel lens 16A is sizedto be about 1.5 square inches and is spaced about 0.6 inches from theillumination source 14. The skilled artisan will instantly recognizethat these dimensions can be adjusted according to particular systemrequirements and are not meant as limitations of the disclosure.

As further shown in FIG. 1, the light 34 passes through the spectralelements 20, which filter out undesired wavelengths to define a desiredspectral region, e.g., 1500-2000 nm, in order to target a particularchemical material of interest. The light 34 is focused by anotherfocusing Fresnel lens 16B, which is also sized to be about 1.5 squareinches and spaced about 1 inch from the chopper wheel 18. As shown, thechopper wheel 18 reflects a portion of the light 34 as a calibration orreference light 35 and a transmitted light 44. The calibration light 35is collimated by a lens 58 before reflecting from a first mirror 24Athrough an adjustable aperture 12B in a bulkhead 12A of the housing 12.The aperture 12B is adjustable to dictate a desired amount of thecalibration light 35. Finally, the calibration light 35 impinges on thebeam splitter 28 thereby sending a portion 35A of the calibration light35 to the first MOE detector 30 and a portion 35B of the calibrationlight 35 to the second or baseline detector 32.

FIG. 1 further illustrates that the transmitted light 44 passes from thechopper wheel 18 into a collimating Fresnel lens 36, which in thisexample is sized to be about 1.5 square inches and is spaced about 0.6inches from the chopper wheel 18. The transmitted light 44 passesthrough another adjustable aperture 12C in the bulkhead 12A and impingesupon a second mirror 24B, which directs the transmitted light 44 towarda sample S in a container C, such as mixing vat or blender. The skilledartisan will recognize that the container C could be a conveyor belt orother device for holding or transporting the sample S and is not limitedto an enclosed container. Likewise, the sample S can be a moving mixturesuch as aspirin and an excipient being blended in real time, a pluralityof tablets passing by on a conveyor belt at high speed, a solution andthe like.

As further shown in FIG. 1, the chopper wheel 18 modulates the lightsignal 44 (between about 50 to about 5000 Hz) to enable thephotodetectors 52, 56 to perform properly. For example, the system 10may be operated with a 10-window chopper wheel 18 rotating at 40 Hz andproviding a chopped signal of 400 Hz. The chopper frequency is chosenbased on several variables, including the rate of motion of the samplematerial S past the sampling window 13, the performance characteristicof the photodetectors 52, 56 and an amplification system, the samplingrate of the data collection and analysis system and the physicalproperties of the chopper motor, control system, and the chopper wheel18 (including window materials).

The number of windows in the chopper wheel 18 can be adjusted to providea desired degree of signal modulation. For example, the chopper 18 mayinclude open windows and black spokes, the latter blocking the light 44.By way of further example, different materials can be placed in thewindows to provide different spectral characteristics for the variouswindows. These window materials may be partially or completelytransmissive to the light signal 44. The transmission characteristics ofthese windows may permit the windows to be used as spectral elements.These windows can also contain MOEs.

With continued reference to FIG. 1, the transmitted light 44 is focusedby the focusing Fresnel lens 26, which in this example may be round andabout 15/16 inches in diameter. The lens 26 may also be adjustablewithin an inner tube 22. Also in this example, the lens 26 may bepositioned about 0.6 inches from an outer surface of the container C.

As further shown in FIG. 1, the transmitted light 44—now focused—passesthrough the transmissive window 13, which in this example isapproximately 1 inch in diameter. Also, an anti-reflective (AR) coatingmay be applied on one or both sides of the lens 26 to ensure that achemical process in the container C does not interfere with themeasuring process of the optical analysis system 10. Thus, thetransmitted light 44 enters the container C and reflects from the sampleS as a carrier light 46.

FIG. 1 further illustrates that the carrier light 46 is directed by thetube 22 in a direction of the first detector assembly 30. Eventually,the carrier light 46 impinges on the beam splitter 28 and a portionpasses in a direction of the detector assembly 32 and the detector 56for baselining with the portion 35B of the calibration light 35. Anotherportion of the carrier light 46 passes through MOE 48, which as notedabove, has been selected for the sample S (chemical of interest) basedon the various components of the system 10. Finally, that portion of thecarrier light 46, having passed through the MOE 48, is focused by thelens 50 and received by the detector 52.

As introduced above, the two signals collected by the detectors 52 and56 can be manipulated, e.g., mathematically, to extract and ascertaininformation about the sample S carried by the carrier light 46. Morespecifically, as the light reaches the beam splitter 28, the light beam34 is divided into a neutral or gray spectrum and a portion of the light(“signal B”) is sent through the lens 54 onto the one detector 56 and aportion of the light (“signal A”) is sent through the MOE 48, throughthe other lens 50 and onto the detector 52. The system 10 measuressignal A and signal B, and a ratio of the two signals can be used tomeasure a concentration of the chemical of interest S. Additionally,monitoring of signal A and/or signal B independently, or in somecombination, can provide other information, such as powder segregation,packing of materials, effect of particle size, etc. Any algebraiccombination of signals A and B can be used; e.g., A and/or Bindependently; A divided by B; A plus B; A minus B; B divided by A; Bminus A, etcetera. For example, a ratio of signal A to signal B canprovide a chemical measurement; individually, A signal and/or B signalcan provide other homogeneity measures including physical make-up of thesample, packing, particle size, and/or separate physical and chemicalproperties.

An auto-calibration process in accordance with the present subjectmatter may be implemented to confirm the signals A and B independentlyor the ratio of A and B. The auto-calibration process according to thepresent technology may be performed according to several differentmethodologies. The following methods are exemplary of the possiblemethodologies and are not intended as limitations on the full range ofmethods that may be employed.

It will be recalled that a portion of the overall system includes achopper wheel as shown in FIG. 1. Rotation of the chopper wheelmodulates the light impinging on the sample and hence the photodetector.A first method of the auto-calibration process involves placing a singleknown material in one or more of the chopper windows. A second method ofthe auto-calibration process involves providing different knownmaterials in several of the chopper windows. In accordance with thefirst and second methods, by having knowledge of the composition of thematerial(s) in the chopper windows, the signal coming from the detectorcan be determined. It should be appreciated that, in general, it is nota requirement of the present technology to provide a specific number ofwindows in the chopper wheel. By using multiple calibration materials inthe chopper wheel, several calibration parameters in the controlsoftware can be set, confirmed, or verified, essentially one percalibration material.

A third calibration method of the auto-calibration process involves amovable mirror (see FIG. 1), positioned so that, either by turning orhorizontal displacement, the light that is normally directed down asampling tube would be directed toward the beam splitter and hence thedetectors without encountering the sample. The mirror can be positionedso that the illumination light beam is directed down the sampling tubetoward the sample focusing lens. During calibration, the mirror isturned toward a second position. In this second position, the light isdirected from the illumination source and the chopper wheel to thebeamsplitter and then to the detectors by way of the beam splitter.

In one embodiment of a movable mirror methodology, a mirror assembly isconfigured to move horizontally with a mirror angled to direct lightdown the sampling tube and a mirror angled to direct light toward thebeamsplitter. During normal sampling, the mirror may be positioned in afirst position and for calibration, the mirror would be moved.

It should be appreciated that it may be necessary to adjust the gain onthe detectors to measure the light from this “bypass.” Alternatively,calibration materials that transmit a lower amount of light can bechosen so that the detectors can be kept at the same gains used formeasuring the sample. Detectors provide an increased output signaldependent upon the amount of light impinging on them. As such, there isa preferred operating region for the detectors and subsequentamplification of the signal such that the final output does vary withthe amount of impinging light. For instance, there are levels of lightthat are too low to produce a reliable output signal. In addition, thereare levels of light that are too great and overload the detectionsystem. At these high light levels, changes in the amount of light donot change the output signal. The preferred mode of operation is wherethe amount of light is in the range where there is a good correlationbetween the amount of light and the output signal of the detectorsystem.

In accordance with the present auto-calibration technology, light isdirected from the illumination sources to the detectors withoutinteracting with the sample. Depending upon the type of sample beinganalyzed and the transmission characteristic of the light path betweenthe illumination source, the sample, and the detectors, there can be arange of signals returned to the detector. As an example, the light pathcould include a fiber optic probe or the sample could be a powder beingmeasured in a reflectance mode. In both of these examples, the amount oflight returning to the detectors during normal sampling could besignificantly less than the amount of light following the by-pass orcalibration route to the detectors. In an exemplary configuration, lighttransmission through a sample may be reduced from 50-99.9%. Thus, inorder to enable the detector and amplification system to operate over auseful range, some attenuation of the signal in the calibration elementsmay be needed.

In accordance with the present auto-calibration technology, a fourthcalibration methodology involves providing an element in a chopper wheelthat turns the light path slightly in addition to having a knownspectral characteristic. Light can be directed to a reflective surfacethat sends light to a beam splitter and then to detectors. A particularaspect to this embodiment is that it allows for a continuous or realtime check of the instrument calibration with each rotation of chopperwheel. In accordance with this method, a stationary mirror assemblyallows the un-deflected beam to pass to the sample for samplemeasurements and the deflected beam to be directed toward the detectionsystem without passing through or encountering the sample.

Turning now to FIGS. 2, 3, 4 and 5, another embodiment of an opticalanalysis system, generally designated by the element number 110, isprovided. Many aspects of the optical analysis system 110 and itsrelated components are similar to the foregoing embodiment; thus, forthe sake of brevity, only certain differences are described below. Toprovide a full and enabling disclosure of the optical analysis system110, when like or similar elements and components are not specificallydescribed below, implicit reference is made to the foregoingdescription.

As shown in FIG. 2, the system 110 is contained in a box or housing 111,which completely encloses the system 110. Thus, the system 110 can beused in a dangerous, explosive environment. In general, a hazard levelof the environment in which the system 110 must operate will determinethe level of containment needed; thus, the housing 111 can be made ofstainless steel, plastic or other desired material as required by theoperating environment. As shown, the housing 111 is divided into a lampside 111A and a detector side 112A, which are described in detail belowwith respect to FIGS. 4 and 5 respectively. As noted with respect tosystem 10 above, the system 110 accomplishes sampling through a windowsimilar to the window W above; i.e., transmissive in a spectral regionof interest.

With reference to FIGS. 3, 4 and 5, detailed views of components of thesystem 110 are provided. FIG. 3 shows portions of the housing 111removed to reveal the lamp side 111A and the detector side 111B indetail. FIG. 4 most clearly shows the lamp side 111A with anillumination source or lamp 114; a lens 116; and a cooling unit or fan153. FIG. 5 most clearly shows a plurality of spectral elements 120; amirror 124; a beam splitter 128; an MOE 148; and lenses 150, 154. Theseand other components are substantially as described above with respectto the foregoing embodiment.

FIG. 6 shows a breadboard embodiment of an optical analysis system,generally designated by the element number 210, which is described byway of exemplary operation below. Many aspects of the optical analysissystem 210 and its related components are similar to the foregoingembodiment; thus, for the sake of brevity, only certain differences aredescribed below. However, to provide a full and enabling disclosure ofthe optical analysis system 210, when like or similar elements andcomponents are not specifically described below, implicit reference ismade to the foregoing description.

As shown in FIG. 6, an illumination source or lamp 214 is providedsubstantially as described above. Also shown are a lens 216A; aplurality of spectral elements 220; another lens 216B; a lens 236; abeamsplitter 228; an MOE 248; a lens 250 and a detector 252. A thirdlens 254 is also shown in communication with a detector 256. These andother components are substantially as described above with respect tothe foregoing embodiment.

The tables shown in FIGS. 7 and 8 list various materials, which can beused in the spectral elements, detectors and windows described herein.For instance, when developing MOEs for a near infrared application of anoptical analysis system, materials with some optically absorbingcharacter may be used as one or more layers of the MOEs. Some examplesof absorbing materials or films are shown in FIGS. 7 and 8, whichinclude but are not limited to, silicon, germanium, sapphire, and dopedversions thereof. Selection of these optical materials depends on thespectral region being measured.

More specifically, measurement precision can be shown to vary from anoptimum value when transmittances are as low as practical to a poorerperformance when overall transmittance is high. Absorbing films make itpossible to engineer optical elements that meet the criteria of havingstrong variations in transmission at the most important wavelengths of ameasurement (determined by a design algorithm) while maintaining lowtransmission at wavelengths that matter least in a measurement. The useof absorbing materials for optical elements thus provides an additionalvariable that can be controlled in the design and manufacture of theelements to reduce transmission in the desired spectral region. With anon-absorbing material, particular spectral regions can be transmissiveor reflective. When an absorptive material is used, an element can bedesigned to be transmissive, reflective, or absorbing in a givenspectral region, which provides expanded design parameters.

With further reference to FIG. 8, spectral ranges of various crystalmaterials are listed. As shown, the crystals have different spectralranges for transmission sampling. In mid-IR, for example, the cutoff atlow wavenumbers varies from approximately 896 cm-1 for CaF2 to 4 cm-1for polyethylene. To some extent, the cutoff values may also be affectedby the thickness of a particular crystal. The skilled artisan willunderstand that to convert from wavenumber (cm-1) to wavelength (μm) asshown in FIG. 8, 10,000 is divided by the wavenumber such that, e.g.,5500 cm-1 is equivalent to 1.8 μm or 1800 nm.

More specifically, measurement precision can be shown to vary from anoptimum value when transmittances are as low as practical to a poorerperformance when overall transmittance is high. Absorbing films make itpossible to engineer optical elements that meet the criteria of havingstrong variations in transmission at the most important wavelengths of ameasurement (determined by a design algorithm) while maintaining lowtransmission at wavelengths that matter least in a measurement. The useof absorbing materials for optical elements thus provides an additionalvariable that can be controlled in the design and manufacture of theelements to reduce transmission in the desired spectral region. With anon-absorbing material, particular spectral regions can be transmissiveor reflective. When an absorptive material is used, an element can bedesigned to be transmissive, reflective, or absorbing in a givenspectral region, which provides expanded design parameters.

With further reference to FIG. 8, spectral ranges of various crystalmaterials are listed. As shown, the crystals have different spectralranges for transmission sampling. In mid-IR, for example, the cutoff atlow wavenumbers varies from approximately 896 cm-1 for CaF2 to 4 cm-1for polyethylene. To some extent, the cutoff values may also be affectedby the thickness of a particular crystal. The skilled artisan willunderstand that to convert from wavenumber (cm-1) to wavelength (μm) asshown in FIG. 8, 10,000 is divided by the wavenumber such that, e.g.,5500 cm-1 is equivalent to 1.8 μm or 1800 nm.

As introduced above, a process for identifying the proper combination ofsystem elements to make an optimally performing system must take intoaccount multiple variables. Specifically, the available types ofdetectors, light sources, optics and filtering elements make it possibleto generate instruments with a wide range of combinations of components.A mathematical analysis can be used to analyze all possible combinationsof these elements with the specific spectral calibration informationavailable for a chemical measurement and render determinations of twoimportant properties before a system is ever built. The results of theseanalyses are then used to determine the overall optical systemconfiguration to use to provide the best performance.

The first of these properties of the complete system is the modelperformance. Model performance is the “best” possible performance for asystem with the components selected in a particular combination. It canbe arrived at by either a conventional multivariate analysis (manuallyor using an automated approach to modeling), or by designingmultivariate optical elements for the specific chemical measurement. Themodel performance can be calculated using the spectroscopic calibrationdata for the system being measured. These calibration data are combinedwith the spectral elements under consideration. A multivariate analysisof the combined data can be used to produce a Standard Error ofCalibration and Standard Error of Prediction. These results can beoptimized within the limitations of the choice of spectral elements todetermine the best combination of spectral elements for the particularmeasurement.

The second of the important properties that can be determinedmathematically is the optimal signal to noise of the instrument. Thiscan be estimated by evaluating the signal levels expected and using thenoise equivalent power of the detectors as the noise level, or—if thesignals are high enough to overwhelm the detector noise with photonnoise—the photon-limited noise. The signal to noise for the expectedsystem can be determined mathematically for the various systemconfigurations under consideration. A system with a high signal to noiseratio will be predicted to have more useful and reliable measurementcapability.

The measurement precision can be estimated by mathematical equation ineither a detector-limited or photon-limited situation. Measurementprecision is given by the standard deviation of the chemical measurementcaused by detection noise, not by model error. The two things—model andmeasurement precision error—work together to define the best theoreticalquality of measurement, and these factors can be estimated withoutactually making an instrument.

Given an estimate of these two important factors, the many possiblecombinations of detectors, light sources, spectral elements and opticalcomponents can be ranked against one another to select the optimalsystem before a system is manufactured. This selection process providesa method to identify the preferred system configuration for making themeasurements of interest.

The embodiments of the disclosure may be better understood withreference to the following example configurations, which are provided toillustrate various aspects of the disclosure but are not intended tolimit the broader scope of the subject matter.

To demonstrate advantages of absorbing films as discussed above, thefollowing configurations utilized silicon as a component in thefabrication of an MOE.

EXAMPLE 1

-   -   The breadboard system 210 introduced above with respect to FIG.        6 was used to measure concentrations of various sample mixtures        5. The system 210 was configured based on the mixtures being        tested. An exemplary configuration of the system 210 included:    -   Illumination: 20 w GILWAY lamp    -   Spectral elements: 5 mm deuterium oxide (D₂O), 5 mm Germanium    -   Optical window: fiber optic probe    -   Detector: InAr detector from Judson    -   MOE: specific to test

EXAMPLE 2

-   -   The system 110 introduced above with respect to FIG. 2 was used        to make measurements on a mixture of aspirin and lactose as well        as various other sample mixtures. The system 110 was configured        based on the mixture being tested. The aspirin/lactose testing        was performed using static testing in which the powdered sample        with a known composition was placed in a dish and the system        light beam was focused on the powdered sample. The output of the        detectors was monitored and recorded. The aspirin/lactose        samples covering the range of 100% aspirin to 100% lactose were        tested. An exemplary configuration of the system included:    -   Illumination: 20 w GILWAY lamp    -   Spectral elements: 5 mm D2O, 5 mm Germanium    -   Optical window: none    -   Detector: PbS detector from New England Photoconductor    -   MOE: specific to test conditions

EXAMPLE 3

-   -   The system 110 was used again to make measurements on a mixture        of aspirin and lactose. The system 110 was configured based on        the mixture being tested. The aspirin/lactose testing was        performed using dynamic conditions in which the lactose powder        was placed in the bowl of a mixer and the measurement system was        attached to the bowl using a Swagelok® brand connector/fitting.        A sapphire window was used to contain the powder in the bowl and        allow the system 110 to interrogate the powder. With the mixer        turning, known amounts of aspirin were added and the system        output signal was monitored and recorded. Aspirin was added in        several allotments to about 37% final aspirin concentration. The        configuration of the system 110 used in this example included:    -   Illumination: 20 w Gilway lamp    -   Spectral elements: 5 mm D2O, 5 mm Germanium    -   Optical window: sapphire window    -   Detector: PbS detector from New England Photoconductor    -   MOE: specific to test conditions

EXAMPLE 4

-   -   A unit similar to the system 110 can be used to perform        measurements of various sample mixtures in both static and        dynamic modes. One configuration of this system included:    -   Illumination: 5 w GILWAY lamp    -   Spectral elements: 5 mm D2O, 5 mm Germanium    -   Optical window: none    -   Detector: PbS detector from New England Photoconductor    -   MOE: specific to test conditions

While the present subject matter has been described in detail withrespect to specific embodiments thereof, it will be appreciated thatthose skilled in the art, upon attaining an understanding of theforegoing may readily produce alterations to, variations of, andequivalents to such embodiments. Accordingly, the scope of the presentdisclosure is by way of example rather than by way of limitation, andthe subject disclosure does not preclude inclusion of suchmodifications, variations and/or additions to the present subject matteras would be readily apparent to one of ordinary skill in the art.

1. A method of developing a multivariate optical element formultivariate optical computing system for use in a predeterminedspectral region, the method comprising: forming an optically absorptivespectral element having an optically absorptive material, the opticallyabsorptive material defining a wavelength-dependent property, thewavelength dependent property being subject to greater variation in thepredetermined spectral region; and disposing the optically absorptivespectral element in the multivariate optical computing system.
 2. Themethod as in claim 1, wherein the optically absorptive material is alayer in the optically absorptive spectral element.
 3. The method as inclaim 1, wherein the optically absorptive material is a film.
 4. Themethod as in claim 1, wherein the optically absorptive material istransmissive.
 5. The method as in claim 1, wherein the opticallyabsorptive material is reflective.
 6. The method as in claim 1, whereinthe optically absorptive material is transmissive and reflective.
 7. Themethod as in claim 1, wherein the optically absorptive material isselected from the group consisting of a silicon, a doped silicon, agermanium, a doped germanium, a sapphire, and a doped sapphire.
 8. Themethod as in claim 1, further comprising selecting the opticallyabsorptive material based on the predetermined spectral region.
 9. Amethod of developing a multivariate optical element for a multivariateoptical computing system for use in a predetermined spectral region, themethod comprising: forming a plurality of optically absorptive spectralelements each including an optically absorptive material defining awavelength-dependent property, the wavelength dependent property beingsubject to greater variation in the predetermined spectral region; anddisposing the optically absorptive spectral elements in the multivariateoptical computing system.
 10. The method as in claim 9, wherein theoptically absorptive spectral elements are transmissive or reflective.11. The method as in claim 9, wherein the optically absorptive materialis a silicon or a doped silicon.
 12. The method as in claim 9, whereinthe optically absorptive material is a germanium or a doped germanium.13. The method as in claim 9, wherein the optically absorptive materialis a sapphire or a doped sapphire.
 14. A multivariate optical computingsystem for use in a predetermined spectral region, the multivariateoptical computing system comprising: a spectral element including anoptically absorptive spectral material, the optically absorptivematerial defining a wavelength-dependent property, thewavelength-dependent property being subject to greater variation in thepredetermine spectral region.
 15. The multivariate optical computingsystem as in claim 14, wherein the spectral element is a multivariateoptical element.
 16. The multivariate optical computing system as inclaim 14, wherein the optically absorptive material is a layer in thespectral element.
 17. The multivariate optical computing system as inclaim 14, wherein the optically absorptive material is a film.
 18. Themultivariate optical computing system as in claim 14, wherein theoptically absorptive material is selected from the group consisting of asilicon, a doped silicon, a germanium, a doped germanium, a sapphire,and a doped sapphire.
 19. The multivariate optical computing system asin claim 14, wherein the optically absorptive material is transmissiveor reflective.
 20. The multivariate optical computing system as in claim14, further comprising: a light source being configured to radiate afirst light along a first ray path; a modulator disposed in the firstray path, the modulator being configured to modulate the first light toa desired frequency; an additional spectral element disposed proximatethe modulator, the additional spectral element being configured tofilter the first light for a spectral range of interest of a sample; acavity being configured to direct the first light in a direction of thesample, the cavity being configured to receive and direct a second lightgenerated by a reflection of the first light from the sample, the cavitybeing farther configured to separate the first and second lights; abeamsplitter being configured to split the second light into a firstbeam and a second beam; wherein the optically absorptive spectralelement is disposed to receive the first beam, the optically absorptivespectral element being configured to optically filter data carried bythe first beam into at least one orthogonal component of the first beam;a first detector mechanism in communication with the opticallyabsorptive spectral element to measure a property of the orthogonalcomponent to measure the data; and a second detector mechanism beingconfigured to receive the second beam for comparison of the property ofthe orthogonal component to the second beam.